Switching power converters include a controller that controls the cycling of a power switch to regulate the delivery of power to a load. During a constant voltage control mode of operation, the controller controls the power switch cycling responsive to a feedback signal derived from the output voltage delivered to a load. The control loop within the controller may be either an analog control loop or a digital one. In a digital control loop, the feedback signal is processed by a voltage sensing circuit including a comparator that drives a binary output signal responsive to whether the feedback signal is greater than or less than a reference voltage signal produced by a digital-to-analog converter (DAC). Since the comparison of the feedback signal and the reference signal occurs every switching cycle, high accuracy and high speed voltage sensing requires a fast comparator and a high resolution DAC.
An example voltage sensing circuit 100 is shown in FIG. 1. To generate the reference signal, a main DAC such as a 9-bit DAC 110 converts a main DAC input signal (D<8:0>) into a main analog output signal. To increase the DAC resolution, a tracking DAC such as a 5-bit DAC 115 generates a tracking analog output signal that is added with the main analog signal in an adder 120 to form an analog reference signal received by a comparator 105 at its positive input. Comparator 105 also receives the voltage feedback signal Vsense at its negative input. A comparator output signal (comp_out) will thus be binary high so long as the analog reference signal is greater than the voltage feedback signal and will be a binary zero signal so long as the voltage feedback signal is greater than the analog reference signal.
It is conventional to form adder 120 using a high-gain and high-bandwidth operational amplifier (op-amp), which requires considerable die area and consumes substantial amounts of current. Accordingly, there is a need in the art for improved voltage sensing circuits having improved density and reduced power consumption.